The present invention relates to data transfer between circuit boards in a card cage, and more particularly to an optical line-of-sight inter-board data transfer.
Generally when two or more circuit boards are mounted in a common enclosure or card cage, there is a need to communicate or transfer data between the boards. The common enclosure usually includes a backplane into which the circuit boards connect, and the data transfer occurs across a data bus incorporated into the backplane. Such data buses use electrical wires to transfer data between the different circuit boards on the data bus. Such interconnections require an electrical connection to the circuit board via either an electrical connector or a more permanent attachment. Applications that require removal of circuit boards from the cage use electrical connectors which are susceptible to damage and can experience failure. Further the data bandwidth of such data buses is limited. Data transmission over electrical backplanes is currently in the hundreds of megahertz. With telecommunications systems reaching data rates in the gigahertz range, the transfer of data across these data buses becomes a definite bottleneck.
The introduction of the fiber optic data bus by replacing the electrical wire with optical fiber has not alleviated all these problems. U.S. Pat. No. 5,488,682 discloses a polymer-based optical connector for interconnecting a plurality of operational modules which has a polymer backplane for supporting a plurality of polymer waveguides on a formable substrate. Each end of each waveguide has to be precisely located adjacent a spherical ball lens located in a precision recess formed in the substrate, with a transparent window mounted opposite each ball lens. In this way light is coupled from fiber on a module or circuit board to fiber on the backplane. This system requires precision alignment. This optical fiber connector, as well as other types of optical fiber connectors and splices that replace the electrical connectors and joints, are also susceptible to physical damage and can experience failure.
Optical data transmission through free-space has been long used to enable communication over varying distances ranging from micrometers in micro circuits to extraterrestrial distances. Free-space is the spatial medium through which light propagates without the aid of a conduit such as an optical fiber, light pipe, wave guide or the like. U.S. Pat. No. 5,224,184 discloses an optical multi-chip interconnect for optically connecting optical signals between chips using interposed lens. The chips transmit light from the edge of one chip to the edge of another via the lens over very short distances—10 to 1000 microns. Generally such devices broadcast to multiple receivers using uncollimated light.
U.S. Pat. No. 5,726,786 discloses a free-space star-coupled optical data bus whereby several subsystems communicate with each other in a broadcast manner using a specified protocol so that only the intended receiver actually accesses the broadcast data. This system requires a certain amount of overhead to process the protocol in order to determine whether the transmitted data is intended for a specific receiver.
What is desired is a method of reliably transmitting data at high rates between adjacent circuit boards in a card cage without using a backplane data bus while allowing the boards to be inserted or removed independently of one another.